1. Field of the Invention
The present invention relates to a computed tomography apparatus for obtaining a tomographic image by acquiring projection data by rotating an X-ray beam around an object to be examined, and performing a reconstruction calculation of the projection data.
2. Description of the Related Art
As an example of an apparatus of this type, an X-ray computed tomography apparatus (to be referred to as an X-ray CT apparatus hereinafter) is known. The X-ray CT apparatus is widely used not only for a medical use but also for an industrial use as an apparatus for obtaining a tomographic image of an object to be examined. In particular, an X-ray CT apparatus in the medical use occupies an important position as image diagnosis equipment. A recent X-ray CT apparatus has a very high scan speed and a short total imaging time as compared to early ones. However, demand has arisen for an imaging operation in a still shorter imaging time, since a short imaging time is required to obtain an image representing movement of a contrast medium injected into a human body in, e.g., a so-called dynamic scan. Therefore, in recent years, an X-ray CT apparatus, which can attain continuous scan using a slip ring, has been developed.
Upon rotation of an X-ray beam around an object to be examined, a helical scan for performing continuous scan by moving the object to be examined in a slice direction (body axis direction) allows a volume scan. According to this helical scan, a large number of tomographic images can be obtained within a short period of time.
A method for imaging slices one by one while fixing an object to be examined in position during rotation of an X-ray beam is called a fixed position scan. For the sake of simplicity, the slice direction perpendicular to a tomographic image, i.e., the moving direction of an object to be examined is defined as a Z axis. A set of projection data belonging to a single fan angle will be referred to as a view hereinafter.
In the helical scan, since an object to be examined is continuously moved in the Z axis direction during rotation of an X-ray beam, the positions of data in 360.degree. views are not constant. Therefore, in order to reconstruct a tomographic image at a certain position, 360.degree. virtual views at that position must be calculated.
In a typical method for calculating virtual views, virtual views are calculated by interpolating 360.degree. projection view data relating to slice positions before and after a slice position of a tomographic image to be obtained, i.e., a total of 720.degree. projection view data. This method is popularly used in clinical examinations. However, since this method uses data for two revolutions of an X-ray beam, and an object to be examined is moved over a considerable distance during this interval, a tomographic image is considerably blurred in the Z axis direction. Therefore, the following method for calculating virtual views from projection view data in a range of 360.degree.+a double fan opening angle is proposed.
The principle of this second method will be described below with reference to FIG. 1. FIG. 1 is a view called a cynogram, which illustrates projection data. In FIG. 1, a detector number (channel number) "n" is plotted along the abscissa, and a view number (projection number) "m" is plotted along the ordinate. Note that a set of projection data belonging to a single projection number, i.e., a horizontal line of the cynogram constitutes a view. In a fixed position scan, if the number of views is sufficiently large, there is a reflection beam with respect to a beam incident on a single detector which is in an opposite direction. Since the reflection beam has the same path in an object to be examined although they have opposite beam directions, projection data and reflection beam data can be regarded as the same data. In a helical scan method, since the bed is moved, no reflection beams are present in a strict sense. However, beams satisfying the relationship shown in FIG. 2 will be referred to as reflection beams hereinafter for the sake of convenience.
Two circles connected by an arrow in FIG. 1 respectively represent projection data and its reflection beam data or reflection view data. Thus, one projection data of virtual views at a slice position can be calculated by interpolation using one projection data and one reflection beam data acquired at the two sides of the slice position. In the second method, as shown in FIG. 1, 360.degree. virtual views at the slice position are calculated from projection data in a region surrounded by a parallelogram of views within a range of 180.degree.+a fan angle 2.alpha. before the slice position, and views within a range of 180.degree.+a fan angle 2.alpha. after the slice position, i.e., views within a range of 360.degree.+a double fan angle 4.alpha.. In this case, all projection data in the region surrounded by the parallelogram are used twice.
Therefore, in the second method, projection data in triangular regions P1 and P2 are used twice although they are farthest from the slice position. Projection data in triangular regions P3 and P4 are not used at all although they have the same projection angles as those of the projection data in the regions P2 and P1, and are closer to the slice position than the regions P1 and P2. As a result, the following drawbacks are posed.
(1) A blur in the Z axis direction cannot be satisfactorily eliminated although it is small as compared to the first method.
(2) A blur in the Z axis direction is non-uniform depending on the pixel positions, and the influence of the blur varies depending on objects to be examined.
According to the X-ray CT apparatus adopting the helical scan, when view groups to be used in reconstruction are sequentially shifted, a large number of tomographic images which are position-shifted little by little can be continuously reconstructed. When such large number of tomographic images are continuously displayed, a pseudo three-dimensional image can be displayed. However, it takes a long time to reconstruct a large number of images one by one. Therefore, demand has arisen for high-speed reconstruction of continuous images in the helical scan.
The applicant of the present invention has already proposed a method of continuously reconstructing a large number of tomographic images according to the first method using 720.degree. views (U.S. Pat. No. 5,315,665).
However, a method of reconstructing continuous tomographic images according to the second method using views in a range of 360.degree.+a double fan angle 4.alpha. has not been proposed yet.
As described above, as a method of obtaining 360.degree. views at a slice position in a conventional helical scan CT apparatus, the first method using 720.degree. views before and after the slice position, and the second method using views in a range of 360.degree.+a double fan angle 4.alpha. before and after the slice position are available. In the second method as well, the influence of a blur cannot be eliminated to a satisfactory level upon calculation of virtual views. Furthermore, the first method includes a method of continuously reconstructing a large number of images, but the second method does not include such a method.
In order to perform continuous reconstruction, a large amount of projection data are required, and a large amount of tomographic image data are generated. Therefore, in order to store these data, the storage capacity is undesirably increased very much. Note that continuous reconstruction is not limited to a helical scan CT apparatus, but can be executed by continuous scan by a fixed position scan CT apparatus. Therefore, a drawback associated with an increase in storage capacity is similarly posed in the fixed position scan CT apparatus.